The present invention relates to a process for the preparation of fluorinated alkanes from alkenes, and more particularly, to a process for the preparation of fluorinated alkanes such as 1,1,1,2-tetrafluoroethane (known in the art as R134a).
R134a has been mentioned as a possible replacement for dichlorodifluoromethane (known in the art as R12) because concern over potential depletion of the ozone layer exists. R12 is used in closed loop refrigeration systems; many of these systems are automotive air-conditioning systems. R134a has properties similar to those of R12 so that it is possible to substitute R134a for R12 with minimal changes in equipment being required. Currently, R134a is unavailable in commercial quantities; therefore, a need exists for a commercially viable process for the preparation of R134a.
Other refrigerants such as 1,1,1-trifluoroethane (known in the art as R143a) are also considered less detrimental to the ozone layer than currently used refrigerants. Currently, R143a is unavailable in commercial quantities; thus, a need also exists for a commercially viable process for the preparation of R143a.
Another refrigerant which is also considered less detrimental to the ozone layer than currently used refrigerants is 1,2-difluoroethane (known in the art as R152). Currently, R152 is also unavailable commercially. Therefore, a need also exists for a commercially viable process for the preparation of R152.
The reaction of organic materials with elemental fluorine has extremely limited utility because fluorine is highly reactive. The fluorination of organic materials with elemental fluorine proceeds spontaneously with explosive rapidity which results in uncontrolled polyfluorination and substrate fragmentation. Because many alkenes are available in commercial quantities, the fluorination of alkenes to produce fluorinated alkanes would be commercially useful. The problem with unsaturated organic materials is that the fluorination reaction is even more violent and dangerous in nature. The fluorination of a double bond evolves large amounts of heat on the order of 107 Kcal per double bond. As a result, breakdown of carbon-carbon sigma bonds occurs and undesired by-products form.
Processes have been designed to eliminate these dangers and drawbacks; unfortunately, these processes suffer from other disadvantages. For example, one approach runs the fluorination at a very low temperature in solvent with a very low concentration of elemental fluorine which is heavily diluted with an inert gas such as nitrogen argon or helium. As a result, this process suffers from an extreme reduction in productivity without alteration of the fluorination mechanism; in other words, regardless of the low fluorine concentration and low temperature, the elemental fluorine still prefers an H abstraction over addition to the double bond.
As an example, Merritt, J. Org. Chem. 31, 3871 (1966) reported on the fluorination of 1,1-diphenylethylene at -78.degree. C. in CCl.sub.3 F using F.sub.2. The three products were 1,1-diphenyl-2-fluoroethylene in 78% yield; 1,1-diphenyl-2,2-difluoroethylene in 14% yield, and 1,1-diphenyl-1,2,2-trifluoroethane in 8% yield. As such, the major product was a fluorinated alkene which resulted from hydrogen abstraction while the minor products were tluorinated alkanes which resulted from fluorine addition to the double bond. This process for the preparation of fluorinated alkanes is commercially undesirable because the reaction must be run at a very low temperature. See also U.S. Pat. Nos. 4,684,452 and 4,754,085.
Although another attempted approach used activated carbon having fluorine absorbed therein for the fluorination of perchloroethylene, benzene, and bromobenzene, the results reported by Watanabe et al., Bull. Chem. Soc. Jpn. 54, 127 (1981) clearly discourage the use of this approach for the addition of fluorine to alkene. The reference reported that no reaction occurred when the activated carbon having fluorine therein was contacted with benzene at 80.degree. C. or bromobenzene at 100.degree. C. The reference also reported that at 80.degree. C., no reaction occurred when the activated carbon having fluorine absorbed therein was contacted with perchloroethylene; at 250.degree. C., only degraded products such as CF.sub.4, CF.sub.3 Cl, CF.sub.2 Cl.sub.2, and CFCl.sub.3 resulted. The presence of these degradation products indicated the rupture of the carbon-carbon sigma bond. Regardless of the temperature used, the results reported in this reference would not lead a person having ordinary skill in the art to use an activated carbon having fluorine absorbed therein for the addition of fluorine to alkenes to form fluorinated alkanes.
As such, a need exists in the art for a commercially viable process for the preparation of fluorinated alkanes, and more particularly, a process for the preparation of R134a, R143a, and R152 wherein the process does not have to be run at low temperatures and a fluorination reaction does indeed occur.